1. Field of the Invention
This invention relates to a continuous process for the manufacture of pigment dispersions and more particularly to a continuous process for the production of pigment in an aqueous dispersion. The pigment dispersion may be used directly in water-based printing inks and coatings and may be used directly in oil-based inks where the associated water can be tolerated. The dispersion may also be used in oil-based printing inks by flushing the aqueous pigment into a water-immiscible printing ink vehicle.
2. Description of the Prior Art
The manufacture of most pigments usually results in the formation of a product of relatively large crystal size or one contianing impurities and has been referred to in the art as crude. This crude required some form of particle size reduction and/or purification to be useful as a pigment having the tinctorial strength and intensity necessary for use in compositions such as inks, paints, etc.
Various methods have been proposed by the prior art to reduce the particle size and improve the purity of the crude.
One such method relates to the milling of the phthalo crude in an organic liquid as exemplified in U.S. Pat. Nos. 2,556,726; 2,556,727; 2,556,728; and 2,908,690.
The use of an aqueous medium for milling crude is known and is disclosed in U.S. Pat. No. 2,999,862. This patent, however, relates to the use of water as the exclusive grinding assistant and results in a flocculated purified product. U.S. Pat. No. 3,775,149 relates to the preparation of pigments and is specifically concerned with the milling of phthalo crude in aqueous suspension, in the presence of a surfactant, to give a filterable paste of phthalocyanine which is predominantly of the beta-pigment form. This method, however, also results in flocculated product where the pigment is purified when recovered as a presscake or in dry form. Thus, flocculation of the pigment has been a desirable feature according to these prior art processes and has provided a means for purifying the pigment. However, the resultant presscake or dry form of the pigment must then be redispersed for use in products such as inks, paints, etc. We have now found that pigment can be produced by a continuous method in aqueous suspension form that can then be used directly in water-based products such as inks and can be directly used in oil-based products where small amounts of water can be tolerated or flushed into oil-based products such as oil-based inks. This aqueous pigment dispersion according to the invention thus eliminates two steps in the use of pigment, i.e., filtering the flocculated pigment and redispersion for an end use.